A man armed with at least 23 guns on a high floor of a Las Vegas hotel rained a barrage of bullets on an outdoor country music festival on Sunday, slaughtering 59 people, including himself, in the largest mass shooting in modern US history.

Nevada retiree Stephen Paddock, 64, killed himself as police stormed his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino, from where he opened fire on the festival attended by 22,000, sending thousands of terrified survivors fleeing.

Police have found no evidence of a motive behind the killings, and no link to international terrorism, despite a claim of responsibility from Islamic State.

Some fleeing fans trampled each other as police scrambled to find the gunman.

More than 500 people were injured and shocked concertgoers, some with blood on their clothing, wandered streets, where the flashing lights of the city's casinos blended with those of emergency vehicles.

Las Vegas casinos, nightclubs and shopping draw some 3.5 million visitors from around the world each year and the area was packed with visitors when the shooting broke out shortly after 10pm local time.

A witness who gave her name as Christine was in tears as she told local TV that she saw a man shot in front of her.

"They were trying to take him out and he was all bloody and unconscious," she said. "My friend was helping him out and they got him out, and we ran."